Make sure that KDE is not configured for multiple desktops.
To configure that, open the KDE Control Center, select
Desktop > Multiple desktops, and set the number
of desktops to 1.

Read the guided tour of xmonad,
so that you'll know what to do once xmonad starts up.
(Ignore the "Starting xmonad" section, though.
These instructions replace that for KDE.)
The xmonad man page
has a nice summary of the default key bindings for xmonad.

5 Sample xmonad configuration for KDE

As usual, place xmonad configuration in ~/.xmonad/xmonad.hs.

The following sample configuration sets up xmonad to cooperate
with the KDE desktop and panel; for more details about how this works, see the
Gnome page.

This configuration also does the following:

uses the Windows key instead of the Alt key as "mod" for xmonad (freeing up Alt for common emacs-style key bindings in applications)

causes certain applications to launch as floating windows

automatically sends certain applications to a specific desktop when they launch.

5.1 Using Config.Kde

For xmonad-0.8 and greater, you can use the XMonad.Config.Kde module to start with good base defaults for xmonad in KDE (Note: KDE4 users with xmonad-0.9 or greater should replace kdeConfig with kde4Config in the following example. The Config.Kde module is found in the XMonadContrib repository, not in the xmonad repository. ) This base config

6 Make xmonad your window manager in KDE

Create the directory ~/.kde/env if it does not
already exist. (For KDE 4 use ~/.kde4/env) Create a file there called set_window_manager.sh
containing only the following line of text:

export KDEWM=/path/to/xmonad

where "/path/to/xmonad" is the path to the xmonad
binary on your system. For example, on Debian systems this
is /usr/bin/xmonad. If you compiled xmonad
from source, it may be something like /home/$USER/bin/xmonad.

You may also have to use chmod to ensure that
set_window_manager.sh is executable.

7 Restart your KDE session

Now end your current KDE session and start a new one.
Welcome to xmonad with KDE!

8 Tips and issues

As in Gnome, it is very important not to use the xmonad mod-shift-q key to exit your session. Use the KDE menu or panel applet. This is dealt with in Config.Kde. It's ok to mod-shift-q to logoutwhen using Config.Kde binding for mod-shift-q.

Since KDE4's system tray is built around Plasma, whose widgets normally live on the desktop layer, it can be difficult to customize the Plasma tray's display for Xmonad. If you use a 3rd party system tray such as Trayer while running KDE, and some or all icons are missing or appear to conflict with the Plasma tray, the conflict might be worked around by disabling the kded status notifier manager. systemsettings -> Startup and Shutdown -> Service Manager -> uncheck "Status Notifier Manager". Additionally, such tray applications might not implement all of KDE's system tray functionality.

To do: In the sample configuration, show how to bindmod-shift-Qto the commanddcop kdesktop default logoutto end the KDE session via a dialog, or to the commanddcop ksmserver default logout 0 -1 -1to end the KDE session directly.